In order to solve the problems of resource depletion and global environment, energy conservation and harvesting of solar energy such as by photovoltaic devices have received much attention. One of the ways of saving energy is to reduce the frictional force of the motion of moving parts used for transportation facilities to thereby save fuel. Many low-friction coatings such as transition-metal dichalcogenides and diamond-like carbon have been developed so far (Non Patent Literature 1). However, the friction coefficients of these solid lubricants usually increase by their reaction with oxygen in air or oil in a high-temperature environment to cause a fatal problem of moving parts. One of the solutions to this problem is using low-friction coatings made from metal oxides. However, the friction coefficient of almost all the metal oxides is large, which therefore makes it difficult to use them as low-friction coatings.
Recently, it has been observed that the friction coefficient in a vacuum of ZnO coatings with crystal preferred orientation is reduced by piezoelectric effect (Non Patent Literatures 2 and 3). The friction coefficient of ZnO coatings was significantly low compared with those previously reported on ZnO by Prasad et al. (Non Patent Literature 4) and Zabinski et al. (Non Patent Literature 5). It is desirable to observe the frictional properties of ZnO coatings with different crystal preferred orientations, in order to understand the low-friction mechanism of the piezoelectric material.
Furthermore, if the frictional force-reduction phenomenon appears not only in a vacuum but in an oil environment, ZnO coatings are expected to be used as a novel low-friction material for energy conservation.